This invention pertains generally to the field of catalytic hydrogenation, and more particularly relates to the catalytic hydrogenation of carboxylic acid groups to hydroxyl groups in hydroxycarboxylic acids.
Dihydroxyalkanes such as ethylene glycol and 1,2-propanediol have uses in a wide variety of applications including as monomers in polyester resins; in antifreeze and deicing fluids; in the manufacture of food, drug and cosmetic products; and in liquid detergents. The demand for 1,2-propanediol has recently increased as it has become more common to substitute 1,2-propanediol for ethylene glycol in these applications.
1,2-propanediol, or propylene glycol, is currently produced by oxygenating propylene to produce the epoxide, propylene oxide. Propylene oxide is then typically reacted with water to form the desired 1,2-propanediol. Because this process begins with propylene, the price of the resulting 1,2-propanediol is linked to the change in the price of oil and other hydrocarbon non-renewable resources. There is a need for a method that produces dihydroxyalkanes from renewable resources such as plants.
It is well known that plants produce glucose from atmospheric carbon dioxide and sunlight in the process of photosynthesis. Because carbon dioxide is a greenhouse gas, any additional removal of the gas from the atmosphere helps to offset the increase in these gases by industrial emissions. It is well known that glucose may be obtained from a variety of natural sources such as corn starch, a natural product obtained from corn. Fermentation of glucose is well known to produce lactic acid, also known as xcex1-hydroxypropanoic acid or 2-hydroxypropanoic acid. In fact, the majority of lactic acid currently produced is obtained through the fermentation of glucose.
Several types of fermentation exist for converting glucose to lactic acid. For example, in homolactic fermentation, the primary fermentation product is lactic acid, and various bacteria such as Lactobacillus delbruckii, L. bulgaricus, L. Leichmanii, L. carsei, and L. salivarus can be used. Surinder, P. C.; Ullman""s Encyclopedia of Ind. Chem., 5th Edition (1990) Vol. A15, 100. In heterolactic fermentation, on the other hand, large amounts of other fermentation products such as acetic acid, ethanol, formic acid, and carbon dioxide may be produced depending on the materials and reaction conditions used. Id.
As non-renewable resources are diminished, the prices of materials obtained from such resources will undoubtedly increase. On the other hand, as advances in fermentation and separation technologies occur, the price of products obtained from fermentation processes will decrease. Thus, the price of lactic acid derived from natural, renewable resources should decrease as these advances are made. Furthermore, as production of glucose and lactic acid increases, the price of lactic acid should drop due to increased competition and economies of scale. Conversion of the carboxylic acid functionality on lactic acid to a hydroxyl group produces 1,2-propanediol. Thus, if an economically feasible method were found that could effect this transformation, a route would be available for producing 1,2-propanediol from a renewable resource. What is thus needed, is an economical method for reducing the carboxylic acid group on hydroxycarboxylic acids to a hydroxyl group.
It has long been known that the catalytic hydrogenation of carboxylic acids is difficult. Thus, reductions of carboxylic acids are usually accomplished through a two-step process wherein the carboxylic acid is first converted into a more readily reducible derivative such as an ester or anhydride. Although the reduction of carboxylic acids has been described, such processes normally employ high hydrogen pressures and are also normally performed in the liquid phase. A process for directly converting a hydroxycarboxylic acid to a dihydroxyalkane, particularly a process which does so at lower pressures, would greatly reduce expenses associated with such a transformation as it would eliminate the unnecessary expenses associated with transforming the carboxylic acid group to a more readily reducible group.
Various patents disclose the reduction of carboxylic acid derivatives. For example, U.S. Pat. No. 2,093,159 issued to Schmidt discloses the reduction of esters to aldehydes and alcohols using activated copper, nickel, silver, zinc, cadmium, lead, or mixtures of these metals. The activating agents disclosed include metal compounds which give acids with oxygen such as chromium, molybdenum, tungsten, uranium, manganese, vanadium, or titanium in addition to compounds of the alkali, alkaline earth and rare earth metals. The patent discloses that metal catalyst activity can be achieved by depositing the metal catalyst on finely divided substrates such as fibrous asbestos, graphite, silica gel or metal powders. The temperatures for the catalytic reduction of esters is disclosed as ranging between 200xc2x0 C. and 400xc2x0 C., and Ni is disclosed as having superior reduction properties over copper.
The catalytic conversion of carboxylic acid anhydrides to alcohols is disclosed in U.S. Pat. No. 2,275,152 issued to Lazier. The catalysts disclosed for use in the reduction include mixtures of difficultly reducible oxides of hydrogenation metals such as chromites or chromates and oxides of magnesium, zinc, and manganese with readily reducible oxides of hydrogenation metals such as those of silver, cadmium, copper, lead, mercury, tin bismuth, iron, cobalt, and nickel. Hydrogen pressure in the process is greater than 10 atm, and operable temperatures are those in excess of 200xc2x0 C.
A process for hydrogenating esters to alcohols with a cobalt-zinc-copper catalyst at temperatures between 100xc2x0 C. and 350xc2x0 C. and pressures ranging from 34 to 681 atm is disclosed in U.S. Pat. No. 4,113,662 issued to Wall. The patent discloses that the cobalt-zinc-copper catalyst is a highly effective ester hydrogenation catalyst in terms of activity, selectivity and stability.
A process for effecting hydrogenolysis of esters is disclosed in GB 2,150,560 issued to Kippax et al. The disclosed process includes contacting a vaporous mixture of an ester, hydrogen, and minor amounts of carbon dioxide with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature ranging from about 150xc2x0 C. up to about 240xc2x0 C. and at a pressure ranging from about 4.9 to 14.8 atm. The addition of carbon dioxide was found to have a profound effect upon the activity of the Cu/Zn hydrogenation catalysts.
The catalytic conversion of carboxylic acids to alcohols has generally been described as more difficult than the conversion of esters to alcohols. Thus, the pressure and temperature required to effect the reduction of carboxylic acids have generally been higher than those required for reduction of esters and other carboxylic acid derivatives.
Catalytic hydrogenation of carboxylic acids and esters is disclosed in U.S. Pat. No. 2,110,483 issued to Guyer. The addition of iron is disclosed as improving the catalytic activity of catalysts, especially copper chromite which is referred to as a particularly suitable catalyst. Metals disclosed as having useful catalytic properties include copper, chromium, nickel, uranium, cobalt, zinc, cadmium, molybdenum, tungsten, and vanadium. The process can be carried out at pressures ranging from 50 to 400 atm and at temperatures ranging from 150xc2x0 C. to 400xc2x0 C.
The reduction of carboxylic acids is also disclosed in U.S. Pat. No. 2,322,098 issued to Schmidt. Suitable catalysts for the catalytic reduction performed at temperatures greater than 120xc2x0 C. and pressures greater than 30 atm, preferably from 100 atm to 300 atm, include copper, nickel, iron, cobalt, and silver. Activated catalysts are disclosed as obtained by depositing the catalytic substance on large surface carriers such as fibrous asbestos, graphite, silica gel, or inert metal powders.
The liquid-phase ruthenium-catalyzed reduction of carboxylic acids is disclosed in U.S. Pat. No. 2,607,807 issued to Furd. The ruthenium-catalyzed reduction is conducted at pressures greater than 200 atm and at temperatures ranging from 90xc2x0 C. to 30xc2x0 C. The patent discloses that the catalytic ruthenium can be deposited on charcoal, and it specifies that the reduction can be performed in batch, semi-batch, or continuous processes.
The liquid-phase reduction of optically active carboxylic acids to optically active alcohols is disclosed in U.S. Pat. No. 5,731,479 issued to Antons. The ruthenium catalyzed reduction is conducted at temperatures ranging from 50xc2x0 C. to 150xc2x0 C. Although the reduction can purportedly be carried out at pressures ranging from 5 to 250 atm, the pressure ranges from only 100 to 200 atm in each of the examples provided.
The reduction of carboxylic acids to alcohols using rhenium is disclosed in U.S. Pat. No. 4,104,478 issued to Trivedi. The liquid-phase reduction is accomplished at pressures greater than 20 atm and temperatures ranging from 170xc2x0 C. to 250xc2x0 C. using rhenium black in combination with ruthenium, rhodium, platinum, or palladium, and the catalysts may be supported. There is no disclosure that the reduction can be performed on carboxylic acids containing hydroxyl groups.
The reduction of carboxylic acids, ketones, and aldehydes is described in U.S. Pat. No. 4,613,707 issued to Kouba et al. The reduction is accomplished with copper aluminum borate at pressures ranging from 68 atm to 340 atm and temperatures ranging from 100xc2x0 C. to 300xc2x0 C.
The reduction of C2 to C12 carboxylic acids at elevated temperatures and pressures using a catalyst with a first component which is either molybdenum or tungsten and a second component which is a noble metal of Group VIII is disclosed in U.S. Pat. No. 4,777,303 issued to Kitson et al. The two-component catalyst may be supported on graphitized carbons, graphites, silicas, aluminas, and silica/aluminas. There is no disclosure that the process can be used to reduce hydroxycarboxylic acids to dihydroxyalkanes.
Thus, a need remains for a low pressure method of converting the carboxylic acid functionality of hydroxycarboxylic acids to hydroxyl groups. More specifically, a need remains for a method for reducing the carboxylic acid functionality of xcex1-hydroxycarboxylic acids such as lactic acid to a hydroxyl group such that 1,2-dihydroxyalkanes are produced.
The present invention provides a catalytic process for reducing the carboxylic acid group of hydroxycarboxylic acids, such as lactic acid, to produce a product having at least two hydroxyl groups, such as 1,2-propanediol. The present invention also provides supported hydrogenation catalysts and the products produced by the catalytic hydrogenation.
The present invention provides a catalytic process including contacting an organic compound, having at least one carboxylic acid group and an xcex1-hydroxyl group bonded to a carbon atom adjacent to the carboxylic acid group, with a catalyst comprising zero valent copper in the presence of hydrogen to yield a reduced product. The carboxylic acid group is converted into a second hydroxyl group and the product thus has at least two hydroxyl groups.
In preferred processes, the organic compound is contacted with the catalyst and the hydrogen at a pressure of less than about 25 atm, more preferably at a pressure less than 10 atm and still more preferably at pressures from about 3 atm to about 7.1 atm and pressures from about 5.8 atm to about 7.1 atm. Other preferred processes are carried out at a hydrogen partial pressure of less than or about 4 atm while others are carried out at a hydrogen partial pressure of less than or about 1 atm.
In other preferred processes, the catalyst is a zero valent transition metal, and in more preferred processes, the transition metal is copper, silver, gold, cobalt, rhodium, iridium, nickel, molybdenum, palladium, platinum, iron, ruthenium, rhenium, osmium, or mixtures thereof. In most preferred processes, the catalyst is zero valent copper.
In some preferred processes, the organic compound is in the vapor phase when it contacts the catalyst, and in other preferred processes, the organic compound contacts the catalyst and hydrogen in the presence of water.
In preferred processes, the product is a 1,2-dihydroxylalkane, and in particularly preferred processes, the product is 1,2-propanediol, ethylene glycol, or mixtures of these. In other preferred processes, the organic compound catalytically reduced is lactic acid, glycolic acid, or mixtures of these.
In preferred processes, the catalyst is a supported catalyst, preferably supported on silica. In particularly preferred processes, the metal catalyst is deposited on a support and the amount of the metal catalyst on the support ranges from about 10 to about 20 weight percent. In particularly preferred processes, the catalyst is supported on silica having hydroxyl groups some of which are capped with hydrophobic groups. The hydrophobic groups are preferably silane or alkyl groups. Preferred alkyl groups include methyl, ethyl, propyl, butyl, pentyl, hexyl, or similar groups while preferred silane hydrophobic groups include trialkylsilanes such as trimethylsilane.
The catalytic process can be carried out at various temperatures such as from about 80xc2x0 C. to about 400xc2x0 C., but is preferably carried out at a temperature ranging from about 125xc2x0 C. to about 250xc2x0 C. In still other preferred processes, the catalytic reduction is carried out at temperatures of from about 180xc2x0 C. to about 250xc2x0 C.
The invention also provides a catalytic process including contacting an organic compound having a first hydroxyl group and at least one carboxylic acid group with a catalyst in the presence of hydrogen at a pressure of less than or about 4 atmospheres to yield a reduced product. The carboxylic acid group is converted into a second hydroxyl group and the product has at least two hydroxyl groups.
The invention provides a supported hydrogenation catalyst that includes a metal catalyst comprising copper. The metal catalyst is supported on silica, the silica having hydroxyl groups, some of which are capped with a hydrophobic group.
In preferred embodiments, the metal catalyst consists essentially of copper. In other preferred embodiments, the hydrophobic groups are alkyl groups or silanes such as those recited above. In still other preferred catalysts, the copper is obtained from copper nitrate.
Further features, and advantages of the present invention will be apparent from the following detailed description.